In general, caustic potash is produced, accompanied by chlorine or hydrogen, through electrolysis of an aqueous potassium chloride solution with an ion exchange membrane method or the like. The thus-produced potassium hydroxide has a good purity, and thus can be used in many fields. Such potassium hydroxide is used as an inorganic chemical material indispensable in daily activities in many fields, for example, production of various types of potassium salts, as well as medical and pharmaceutical products, cosmetics, and analyzing reagents. Among those, a highly pure product from which impurities are removed to a specific degree is required in such fields as battery/cell materials, electronic materials, and medicine. In recent years, highly pure caustic potash containing an extremely small amount of sodium components and impurity metals has been required for chemical mechanical polishing of silicon substrates for use, for example, in LSIs, in order to prevent impurities from entering and contaminating the silicon substrate to be highly integrated. Further, it is necessary, for example, in alkaline batteries/cells to prevent the stability in terms of voltage and the like from lowering over a long period of time, and prevent liquid from leaking, due to heavy metal components or chloride ions in caustic potash. In view of the above, a highly pure caustic potash product has been in demand.
In response to this demand for increased high purity in caustic potash, there are disclosed methods for purifying potassium hydroxide in which an electrolysis chamber is modified or crystallization is used (see, for example, Patent Documents 1 and 2). Further, there is also disclosed a method for obtaining highly pure caustic potash by purifying potassium chloride before subjecting to electrolysis (for example, Patent Document 3). However, in the methods for obtaining caustic potash through electrolysis, low sodium caustic potash can be obtained using low sodium potassium chloride as a material to be treated, but it is necessary to obtain a large amount of low sodium potassium chloride. Therefore, the methods for obtaining caustic potash through electrolysis are disadvantageous in view of costs, in comparison with purification through crystallization. Further, crystallization through cooling is disadvantageous in view of costs, in comparison with crystallization from a high temperature state, taking concentrating and cooling of the mother liquor into consideration.
Further, as a purifying method utilizing, for example, the difference in solubility, there is disclosed a method for increasing the purity of aluminum chloride, by repeating a crystallization operation in many steps in which crystallizers are arranged in series (for example, Patent Document 4). The costs for purification in this case greatly differ, depending on the yield of the purified product, in addition to the properties of the purifying method.
It is known that such metal components as iron and nickel remain on the surface of a silicon wafer when the silicon wafer is etched, thereby to change the electrical properties. Therefore, it is desired that the content thereof is low. Thus, there is disclosed a method for reducing the nickel content to 0.05 ppm or lower, using a filtering apparatus where activated charcoal is pre-coated (for example, Patent Document 5). Further, there is disclosed a method for reducing the iron content to 200 ppb or lower and the nickel content to 20 ppb or lower, by bringing caustic soda into contact with activated charcoal that has been activated through immersion in nitric acid (for example, Patent Document 6).
Patent Document 1: JP-B-3-061605 (“JP-B” means examined Japanese patent publication)
Patent Document 2: JP-B-5-082328
Patent Document 3: JP-A-2002-317286 (“JP-A” means unexamined published Japanese patent application)
Patent Document 4: JP-A-2004-203713
Patent Document 5: JP-A-2000-203828
Patent Document 6: JP-A-2005-001955